Example : at one point it was seen as obscene to talk about copulation, coit and fornication. So, for the houses where the king approved fornication, they simply created a modest acronym F.U.C.K. (short for : Fornication Under King’s Consent) that later became itself seen as (of course) obscene.

I believe that theory has been debunked and 'fuck' is from low German, meaning 'to strike'.